When comparing Skeleton vs PostCSS, the Slant community recommends PostCSS for most people. In the question“What are the best minimal CSS frameworks?”PostCSS is ranked 5th while Skeleton is ranked 8th. The most important reason people chose PostCSS is:

PostCSS allows you to opt-in to the features you need with plugins. This allows you to set it up to behave exactly like Sass, with nesting, mixing, extends, and more. On the other hand, it allows you to use plugins by themselves for things like auto-prefixing, minification, and more. You can even set up your own custom "stack" of plugins to do exactly what you like.

Pros

Pro

Lightweight

Pro

Responsive grid

Pro

Style agnostic

Pro

Vanilla CSS

No bells and whistles for Skeleton, it's just CSS.

Pro

Media queries

Pro

Flexible

PostCSS allows you to opt-in to the features you need with plugins. This allows you to set it up to behave exactly like Sass, with nesting, mixing, extends, and more. On the other hand, it allows you to use plugins by themselves for things like auto-prefixing, minification, and more. You can even set up your own custom "stack" of plugins to do exactly what you like.

Pro

Doesn't force designers to learn a new syntax

Rather than learn a different syntax, PostCSS allows you to write in pure CSS.

Pro

Fast

PostCSS is 3-30 times faster than Sass (including libsass), Less, and Stylus

Pro

JavaScript-based out of the box

Since it's basically CSS extended through JavaScript it works in the browser directly without the need to compile it beforehand.

Cons

Con

Not maintained anymore

No active development for two years.

Con

Harder to install and keep working

The immense flexibility of PostCSS plus its current rapid evolution makes it harder to install, configure and keep running than the more monolithic and mature preprocessors.

Con

Some plugins need to run in a certain order

Some plugins can only work if initialized after some other plugins. For example, transforming and applying CSS variables needs to run before running a plugin which uses these variables inside conditional transformations.